The present invention is directed to a novel method of injection molding ceramic compositions into ceramic articles.
The ceramic materials are of critical importance for a number of high temperature, high performance applications. Recently, there has been substantial interest in the development of ceramic compositions for critical engine parts including reciprocating engines, gas turbine and rocket engines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. However, the inability to produce complex shapes of high dimensional accuracy and sufficient strength using an economical fabrication technique has prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
Several processes have been used in an attempt to form ceramic bodies. Among such processes include pressing ceramic powder into a greenbody followed by sintering or by hot pressing and subsequently shaping or machining the sintered body to produce the finished product. Another technique is slip casting in which the ceramic particles are dispersed in water, the slurry placed in a mold and the water removed to form a greenbody. The pressing techniques have been found unsuitable to form ceramic articles of complex shapes and which must meet specific design specifications. The slip casting technique is time consuming and has not yielded greenbodies of sufficient strength. In view of the problems associated with the prior techniques, the injection molding process has been increasingly used to form ceramic articles. Injection molding is a process wherein a moldable composition is forced into a mold or die. The injection molding process facilitates a rapid and repeated forming of a plurality of articles having a consistency with close dimensional tolerance. The injection molding process minimizes the amount of shaping or machining that may be required to produce a finished article.
Thermoplastic injection molding is a widely practiced fabrication technique for forming solid plastic articles. The articles are formed by injecting a molten thermoplastic polymer into a mold that is held at a temperature below the freezing point of the polymer to freeze the polymer in the mold. Filled polymers, typically having 10 to 40 volume % filler, are also frequently injection molded with success. Injection molding of highly-filled (i.e., 40 to 87 volume % filler) is less common and more difficult due to the much higher viscosity of the highly filled thermoplastics.
Thermoplastic injection molding as above described has been adapted for use in forming dense ceramic shapes. The process involves forming a ceramic greenbody by injection molding a composite comprising ceramic powder dispersed within a thermoplastic polymer, burning out the polymer, and sintering the resulting porous greenbody to a dense ceramic part with the same shape. The thermoplastic acts as a fluidizing agent to distribute the injection pressure throughout the mold and as the material which holds the ceramic particles in the shape of the mold after the part is ejected. A typical ceramic powder/thermoplastic polymer composite has a very high content of the ceramic particles, typically from about 50 to about 87 volume % and a minimum of the binder material to the hold the particles together in desired shape. Besides the use of thermoplastic binders, it has been suggested to use thermosetting resins as the binder for the ceramic particles. The thermosetting resin is cured or cross-linked in the mold. For example, U.S. Pat. No. 4,627,945 is directed to injection molding refractory compositions utilizing thermosetting phenolic resins which are cured in the mold by the addition of a curing agent to the mix of ceramic and thermosetting binder.
Besides the major binder component which comprises the thermoplastic or thermosetting resin, other additives are typically included in the ceramic/plastic composite which is injection molded. Thus, a typical composite will also contain a minor binder component which is often a thermoplastic, wax or oil and is removed early in the binder removal cycle, plasticizers which increase the fluidity of the ceramic-binder mixture, and processing aids such as surfactants which improve the wetting characteristics between the plastic binder and ceramic during mixing to form the composite.
A summary of injection molding applied to the fabrication of molded ceramic bodies is provided in an article entitled "Review: Fabrication of Engineering Ceramics by Injection Molding. I. Materials Selection", M. J. Edirisinghe et al, International Journal of High Technology Ceramics, Vol. II, 1986, pp. 1-31.
Various difficulties have been encountered utilizing injection molding as a route to the formation of dense ceramic parts. Specifically, the highly-filled ceramic powder-polymer mixes which are needed to produce a dense ceramic article are also excessively viscous. Good flow during injection molding requires a mix viscosity of less than 1,000 Pa.s (104 poise) in the shear range of between 100-1,000 Sec.sup.-1 usually encountered in the gates and mold. To confer this fluidity to the ceramic-binder mix, the art has found it necessary to reduce the ceramic powder filling level. At reduced ceramic powder filling levels, the mix has sufficient fluidity to be injection molded, but the greenbody has an excessive level of binder. Accordingly, the conversion to a dense ceramic is accompanied by larger dimensional changes and is prone to higher instances of defects. A highly viscous resin binder, on the other hand, makes it more difficult to uniformly disperse the ceramic powder therein to form a uniform ceramic powder-plastic composite. Related to this is the difficulty in monitoring or characterizing the state of dispersion of the ceramic particles in the resin binder. Nonuniformities within the composite can result in the formation of fractures in the ceramic greenbody and/or the finished ceramic article.
Another problem associated with injection molding ceramic greenbodies from ceramic-binder mixes which utilize thermoplastic binders is that the thermoplastic binder must be melted to achieve the minimum mix viscosity. Thus, injection molding ceramic-thermoplastic binder mixes involves heating the extrusion device which mixes the ceramic powder and binder and eventually injects same into a cooled mold. Mixing and dispersing the ceramic powder uniformly into the binder and injection of the mix into the mold constitutes the majority of the injection molding process and, thus, consumes the greatest amount of energy. Moreover, to clean the extrusion device requires a shut down of the device and the eventual cooling thereof. Upon cooling, any thermoplastic polymer which remains adhered to the walls and screw of the extruder solidifies. Elaborate means are necessary to scrape the polymer off the internal surfaces of the extrusion device.
To overcome the problems associated with the injection molding of plastic parts which are caused by the high viscosity of the polymers which are used, reaction injection molding (RIM) has been utilized. The molding of plastic parts by RIM is a recent development. Plastic RIM involves injecting a reactive, low viscosity, liquid monomer or combination of monomers into a mold and causing the liquid to solidify and take the shape of the mold by initiating the polymerization of the monomer liquid in the mold itself. The objective is to produce a plastic part in the shape of the mold that is strong and rigid enough to be ejected from the mold. As far as is known, it is not believed that reaction injection molding has been used in the production of ceramic greenbodies.